Which configuration is safer in collisions when you disregard ease of installation? I posed this question to Robert Bell who has worked in the child car restraint industry for more than 35 years. This is his reply.

Posted here with kind permission form Robert himself:

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Hi Lena

The answer is both easy and a little complicated. The basic idea with ISOFIXis to reduce the risk of misuse, and we have a few studies that make uspretty sure that ISOFIX systems do get installed correctly more often. As aresult, they are better in crashworthiness.

But even if we disregard misuse, there are some reasons for believing thatthe actual performance of the product improves. The basic goal for childrestraints is to restrain, to keep the child from moving in the vehicle,while the vehicle structure itself crumples and absorbs energy, retardingthe movement of the passengers in a way which the automotive engineersdesign to yield the best chances of reducing injury. You tie the passengersas tightly to the car chassis as you can, and you let the chassis be theprimary means of protecting the occupants.

This is one of the ways rearward facing systems outperform forward facingsystems, entirely ignoring the issue of head and neck loads. The forwardfacing system, typically anchored to seat belts, moves forward in a crash asthe seat belts extend, the plastic in the seat itself stretches, the childstretches the internal child harness in the seat and the vehicle seatcushion collapses under the load of the seat. The outcome is that the childmoves forward, close or beyond the 550 mm extension that is allowed in thestandard. What this means is that during perhaps a third of the crash thechild cruises, motion unchecked, forward in space, and the retardation endsup taking place in a shorter period of time, resulting in a higher load onthe child during the 40 ms or so that remain.

This is just physics, and the rearward facing system exploits the physics ofthe vehicle better.

The rearward facing seat, when placed on dash, moves very little in thecrash, perhaps as little as a centimeter or two and as a result the child isretarded throughout the entire ride down. The loads on the child aretherefore much lower than they would be in a forward facing system. It isalso important to remember that the loads on the fragile neck and head arereduced dramatically in relation to forward facing systems. But one of theadditional big advantages of those rearward facing systems is that theyrestrain more effectively. Rearward facing seats in the rear aredisadvantaged by the rear seat back, which moves more than the dashboard,but they are still superior to forward facing CRS in this regard.

Now you can improve both systems with ISOFIX. But the improvement is notvery dramatic for a rearward facing system, particularly one that isinstalled on dash in the front seat. A forward facing system, on the otherhand, is much more tightly tied to the vehicle in the case of a Europeanstyle hard ISOFIX installation, when compared to the traditional seat beltinstalled versions. It is also important to remember the contribution of theupper tether/ floor support in reducing the motion of these ISOFIX forwardfacing systems. Some of these features have also migrated to belt installedsystems, so they get a least part of the improvement.

Lastly, in studies performed by the UK Transport Research Lab, improvementswere noted in side impact performance for various types of ISOFIX seats. Thereason is most likely linked to the lateral stiffness provided by the hardEuropean ISOFIX anchors. These seat move less in side impact and aretherefore likely to perform better in the real world.

How is all this expressed in actual crashes? In the case of the rearwardfacing systems we will be hard pressed to note improvements. Excepting firesand the like, and accidents in which the car is crushed flat by a lorry orcut in two by a tree, we never see fatalities in the seats now in use inSweden. Most every summer we have a dramatic crash in which families aredecimated in frontal accidents, except for the toddlers that are seated inrf systems. We had one this summer in Piteå, parents killed, two siblings,one killed and one seriously injured, rear facing toddler unharmed. So wedon’t expect major improvement in the real world of rear facing systems asISOFIX becomes more common.

In jurisdictions with forward facing systems, I expect that we will see areduction of fatalities and serious injuries among children. Mostspecifically, I think we will see a reduction in compressive head and neckinjuries, as these may occur in frontal crashes when the child is flungforward and impacts the back of the front seats with the head. ISOFIX seatswill reduce the chance of that sort of impact and the resultant injury. Myguess is that ff systems will be very much improved in the real world by theincreased use of ISOFIX equipped CRS.

Does this make sense to you? Let me know if you have questions on thisscore.